The advantages of locating a complete sewage processing facility offshore to accommodate the sewage waste disposal requirements of an adjacent community, have been recognized in the prior art. Patents to Martineau U.S. Pat. No. 5,032,289, Kantor U.S. Pat. No. 4,944,872 and Hammel U.S. Pat. No. 4,692,249 exemplify apparatus and methods heretofore proposed for offshore treatment of sewage, in which primary and secondary sewage treatment facilities have been proposed for mounting on a moored vessel or offshore platform, in which waste water treatment and disposal, digestion of solids and their dehydration and disposal, are described.
Development of such proposals has been restricted by economics of scale and lax or non-existent environmental requirements. Further, none of the prior art systems disclose designs capable of handling peak volumes of sewage or accommodating plant overload conditions when storm sewer runoffs are tied into sanitary sewer systems, thereby producing combined sewer outflows ("CSOs").
When the CSO's exceed the treatment capacity of the prior art systems, such excesses have heretofore of necessity been deumped into the marine environment, causing pollution. The Marineau U.S. Pat. No. 5,032,289 describes an offshore sewage treatment facility with this limitation.
Additionally, technical problems arise from support and structural difficulties when extremely heavy loads from waste-filled treatment facilities are installed on a floating or fixed offshore structure.
Floating installations in which the tankage volume is totally supported by a water volume contained within an exterior floating unit are already known. Japanese patent 59053296 to Hitachi Zosen discloses a double-hulled structure in which an interior floatable vessel used for plant-support, is floated within an exterior floatable vessel, the system being adapted for movement to a suitable location at which the exterior floatable vessel is sunk to the sea bottom, leaving the interior plant-supporting vessel floating within the gravity-based exterior vessel.
U.S. Pat. No. 4,217,848 to Meyer-Haake discloses a double hulled system in which the interior tankage is used as a floating gas liquefaction unit, supported within an annular floating unit, both units being floated independently of one another in a marine environment.
In each of these prior art descriptions, the operative vessel is totally and independently floated and utilizes the exterior vessel in a minor and ancillary role.
European patent 0 165 328 to INTEKA S.A. discloses a sewage treatment system in which the primary sewage treatment is performed on a land-based facility, following which the discharged effluent is pumped into a secondary clarification system, the latter being installed in a converted oil tanker. Within the oil tanker, the effluent is aerated and contained sludge is settled in a secondary on-board system of tanks, each of these being structurally supported from the underlying hull of the oil tanker. Cleaned effluent is then discharged for disposal into the marine environment.
None of the prior art references disclose means for float-supporting a weighted vessel which is structurally fixed in an interior water body, for either partial or total float-support of the weighted vessel against gravity forces.